1. Field of the Invention
The present invention relates to an apparatus configured to read a license plate automatically from an image, a method and a program therefor.
2. Description of the Related Art
Conventionally, a fixed camera provided at a position above a road photographs a vehicle in order to read a license plate of the vehicle for the purposes of traffic control. For example, it is possible to read a license number of the vehicle passing through the plurality of road points and obtain a travel time and/or an average speed between road points by searching for an identical license number from a list of read results at the respective road points. In addition, this method can be used for an automated toll collecting system for a toll road.
When a license plate is read using such a fixed camera, since the license plate is photographed from the same angle, to a certain degree with a similar background, the license plate can be detected with relatively high accuracy.
In addition, it is discussed to photograph a plurality of images and to recognize the license plate that seem to belong to a same vehicle. When the plurality of images is identical with each other, the result is regarded as a final recognition result so that a recognition rate is improved (Japanese Patent Application Laid-Open No. 9-288795).
In addition, Japanese Patent Application Laid-Open No. 11-073514 discusses a technique for detecting a license plate region candidate by detecting a straight line of a frame of the license plate from an image using Hough transform and recognizing characters by performing a keystone correction.
In addition, Japanese Patent Application Laid-Open No. 6-215293 discusses a technique for obtaining a vertical edge image by differentiating in the horizontal direction and detecting a position of a license plate from the edge image.
However, it is not easy to detect the license plate from an image photographed by an unfixed camera such as a commercially available digital camera because a photographing condition (e.g., photographing angle, luminance, background) thereof is variable. In addition, miscellaneous images may be included in the images photographed by the unfixed camera in addition to the license plate. For instance, if an object similar to a license plate is included in a photographed image, it becomes difficult to determine whether the object is the license plate.
For instance, when trying to detect a license plate region by detecting a frame of the license plate, a region having a rectangular frame such as a headlight part or a front grill of a vehicle may be incorrectly detected as the license plate region. In addition, when characters are recognized while detecting character blocks, for instance, commercial message characters written on a body of commercial vehicles may be incorrectly recognized as license plate characters. In cases where characters on signboards of shops in the background are photographed in an image, character strings of these signboards may be incorrectly detected as the character strings of the license plate.
In addition, when a user photographs a vehicle using a digital camera or the like, the same vehicle is not always photographed several times. Further, there is a possibility that the license plate is photographed from various angles.